Engineered Lithium Fluoride Layer Enables Fire-Safe, Long-Lasting Lithium Metal Batteries

A new study demonstrates how a precisely engineered lithium fluoride layer can protect lithium metal anodes from corrosive flame-retardant additives, enabling the creation of batteries that are both inherently fire-safe and capable of long-term, stable operation. Published in the journal Carbon Energy, the research addresses a critical barrier in battery development where safety enhancements typically come at the expense of performance and longevity. Lithium metal anodes offer exceptional theoretical energy density but are plagued by dendrite growth, unstable chemistry, and the high flammability of conventional liquid electrolytes.

While gel polymer electrolytes improve safety, they often require large quantities of organic phosphate flame retardants like triphenyl phosphate. These additives, however, penetrate the battery's natural solid electrolyte interphase, triggering decomposition reactions that severely corrode the lithium anode and drastically shorten battery life at high concentrations. The research team tackled this problem with a dual-strategy approach. First, they engineered a flame-retardant gel polymer electrolyte with a high 70 wt.% TPP loading using a coaxial electrospinning technique. This created a dual-confinement structure with a TPP/PVDF-HFP core surrounded by a PAN/PVDF-HFP shell. As detailed in their study(DOI: 10.1002/cey2.70077), strong chemical interactions and physical containment within this design maintain flame retardancy while limiting the corrosive side reactions typically caused by TPP.

The second component involved pre-forming an artificial, LiF-rich SEI on the lithium metal anode by immersing it in a 5% fluoroethylene carbonate-containing electrolyte. Advanced characterization techniques including UV–vis spectroscopy, TOF-SIMS, XPS, and AFM confirmed that this engineered SEI layer effectively blocks the penetration of TPP-derived corrosive species and significantly reduces the depth of anode corrosion. Beyond providing protection, the LiF layer enhances lithium-ion mobility, lowers activation energy for interfacial transport, and promotes smooth, dendrite-free lithium plating. Electrochemical testing validated the combined approach's effectiveness. Lithium symmetric cells operated stably for 2400 hours at 0.5 mA cm⁻² and 1500 hours at 5 mA cm⁻².

In full-cell configurations with lithium iron phosphate cathodes, the batteries retained 98.9% capacity after 1500 cycles at 1 C and maintained 81.7% capacity after an extraordinary 6000 cycles at 10 C, demonstrating exceptional endurance under fast-charging conditions that typically degrade battery performance. The lead corresponding scientist noted that the study shows precise interface engineering is essential for advancing both safety and durability in lithium metal batteries. By integrating the dual-confinement flame-retardant electrolyte with the LiF-rich artificial SEI, the researchers resolved the fundamental conflict between fire protection and anode stability. This approach not only halts severe corrosion from phosphate-based additives but improves lithium-ion transport, enabling reliable operation under high rates and extended cycling.

This combined SEI-electrolyte strategy represents a promising direction for developing high-performance, intrinsically safer lithium metal batteries suitable for electric vehicles, grid-level storage, aerospace systems, and next-generation flexible pouch cells. The underlying design principle of merging chemical confinement, structural encapsulation, and deliberate SEI engineering could potentially be applied to other reactive anodes and high-voltage cathodes. As global demand for high-energy batteries intensifies alongside increasingly strict safety requirements, this approach may accelerate the practical adoption of lithium metal technologies that have long been limited by safety-performance trade-offs.